The present invention relates to a method for securely monitoring a distance between a metallic part and an inductive proximity sensor, in particular for monitoring a closed position of a guard door in an automated system.
The invention also relates to a signaling device for securely monitoring a distance between a metallic part and an inductive proximity sensor, in particular for monitoring a closed position of a guard door in an automated system.
In order to make safe a hazardous area of a system which operates in automated fashion, for example robots, presses or conveyor belts which operate in automated fashion, guard doors are often provided, which doors prevent the operator from coming into contact with those parts of the system which may be hazardous to the operator.
So that the guard door can fulfill its protective function, before starting the system it has to be verified that the guard door is properly closed. For this purpose, an inductive proximity sensor of the type described below may be used which checks whether the guard door is at a particular position. The system cannot be switched on as long as the required position of the guard door has not been reached.
The inductive proximity sensor operates according to the following principle. The proximity sensor contains an oscillating circuit which is caused to oscillate by means of a pulse. The oscillating circuit has a characteristic decaying oscillation if a metal to be detected is in the vicinity thereof. If the metal then approaches the proximity sensor, the attenuation and/or frequency of the decaying oscillation change(s).
However, the fact that the metal to be detected must be known in advance and the proximity sensor must be constructed in an appropriate manner gives rise to difficulties in this case. If, for example, the intention is to detect a particular distance both for aluminum (non-ferromagnetic) and for iron (ferromagnetic), either a reconfiguration or a very complex device for determining the material is required. Only after the material is known can the distance then be determined. Nevertheless, even in the case of a material detection, which can distinguish between ferromagnetic and non-ferromagnetic materials, problems remain in the case of metal alloys which have both ferromagnetic and non-ferromagnetic components.
In addition, it has been found in practice that the operations of opening and closing the guard door, for example, in order to clamp a workpiece, are found to be a nuisance by some operators. In the past, it has therefore been the case that operators have manipulated the safety device by fixing a piece of metal directly to the proximity sensor. In this case, the system could be operated even with the guard door open since the sensor incorrectly detected the piece of metal as a closed guard door.
In order to counteract such attempts at manipulation, it is desirable to determine or monitor the distance between a proximity sensor and a metallic counterpart in such a manner that the system is also not switched on or is switched off when the distance of the metal to be detected is too small.
DE 102 22 186 C1 discloses a safety switch having two transmitting/receiving elements. In this case, one element is fitted in a stationary manner, for example, while the other element is fastened to a guard door. In a first step, the first element transmits electromagnetic signals to the second element which is in the form of an LC-oscillating circuit. As a result, the second element is caused to oscillate and stores a part of the transmitted quantity of energy which is dependent on its distance from the first element. The energy stored in this manner is then emitted in the form of electromagnetic waves at the frequency of the LC-oscillating circuit of the second element. A part of this energy which is again dependent on the distance between the two elements is then transmitted back to the first element. A quantity of energy which is a measure of the distance between the two elements is generated in the first element by means of integration over a predetermined period of time. If the quantity of energy received differs from an expected quantity of energy, a state is assumed in which operation of the corresponding system is not allowed.
EP 1 278 077 A2 discloses the general principle of an inductive proximity sensor which does not depend on an appropriately designed counterpart (transmitting/receiving element). However, the sensor does not have the protection against manipulation required for use in safety technology.
Against this background, an object of the present invention is to show a cost-effective device and a method for securely monitoring a distance between a metallic part and an inductive proximity sensor even if the approach of different metals needs to be taken into account. In this case, the intention is to be able, in particular, to protect against manipulation in a manner which ensures compliance with a minimum distance and allows the same minimum distance for different metals.